Electrolyzing of chrome solutions to recover chrome



Patented Apr. 11, 1950 ELECTROLYZING OF CHROME SOLUTIONS TO RECOVER CHROME Dmitri Vedensky, Berkeley, Calif., assignor to Pacific Bridge Company, a corporation of Delaware Application January 10, 1947, Serial No. 721,425

3 Claims.

This invention relates to the electrolysis of water soluble chromate solutions to produce chromium metal.

I have discovered that a water soluble chromate solution can be successfully electrolyzed to produce valuable chromium metal if (a) the pH of the electrolyte is maintained between pH 4 and pH 7 and (b) the solution is subject to electrolysis in an electrolytic cell which includes mercury as a cathode.

During the electrolysis, the alkalinity of the electrolyte in the cell tends to increase. If this is permitted, the rate of metal formation decreases until it may cease and only chromium hydroxide is formed concurrently with a decrease in current efficiency. It is therefore essential that the electrolyte be maintained in the range of pH 4 to pH 7 by addition of a suitable acidic medium, preferably one which does not add any interfering component or radical to the cell; such an acidic medium is chromic acid. The electrolyte can also be constantly replenished with fresh acid electrolyte to maintain it in the range of pH 4 to pH '7.

My investigations have indicated and shown that the use of mercury as a cathode is a critical factor in such electrolysis inasmuch as if one employs any other metal as a cathode, electrolysis of the solution either does not proceed or takes place at a greatly reduced efficiency. When an electrolytic cell is operated to produce chromium metal, the metal is not immediately available as such for it forms an amalgam with the mercury. From this the chromium can be recovered by heating the amalgam in a retort to vaporize the mercury which is subsequently recovered. The chromium remains behind in the retort as substantially pure chromium free of mercury and other contaminants, particularly iron, it only being necessary to heat the amalgam in a furnace in which a non-oxidizing or a reducing atmosphere is maintained to ensure that a chromium oxide is not formed. 1

I have also discovered that the presence of certain impurities in the electrolyte is detrimental to the electrolysis, although the process is not as sensitive to small amounts of impurities as are, for example, zinc or manganese electrolytes. However, if the amount of any impurity becomes excessive, it is necessary to resort to a purification step to precipitate the impurity as an insoluble compound and remove it from the electrolyte. Since one application of the process is in recovering chromium metal from chromate plant solutions, the usual impurities are tho e which may be found in chrome ores and which are extracted as sodium salts by the water leaching which follows the customary soda roast.

I have investigated several elements which can be found in chromate plant solutions by conducting electrolysis in the usual manner using pure chromate solutions containing 55 gms. Cr/liter, to which known amounts of impurities were added.

The following table shows the results:

Power Con- Amount Impurity Added Added, fgfi tfg g mgfllter Or Produced Vanadium was of particular interest during this investigation because plant solutions which were used contained vanadium in amounts exceeding 160 mg./liter. Until a successful method of eliminating vanadium was established, successful electrolysis of chromium could not be conducted upon a solution of crude sodium chromate such as would result from the soda roasting of a chrome ore.

I have found that satisfactory purification of electrolyte can be accomplished by precipitating the vanadium present as an insoluble salt. One can employ any water soluble reagent which forms a vanadium salt which is relatively insoluble in the electrolyte and a chromate which is soluble in the electrolyte. I have successfully used calcium and iron. In one test, lime was employed in the amount of 0.05 of CaO per pound Cr present, heating the solution to C. and filtering 01f precipitated calcium vanadate and excess of calcium hydroxide. Using this procedure, a valuable vanadium by-product was obtained, i. e., calcium vanadate, while the electrolysis of the purified liquor was restored to normal conditions and the power consumption reduced to 14.7 kWh/pound and 13.2 kwh./pound of Cr deposited in two different runs.

The process of this invention will become further apparent upon considering the following more specific examples in connection with the drawings wherein Figures 1 and 2 illustrate diagrammatic arrangements of suitable apparatus which can be employed and suitable flow sheets.

Referring to Figure 1 in the drawing, I have indicated an electrolytic cell 6 provided with an anode l in the form of a horizontal perforated plate. The cathode is provided by a pool of mercury indicated in the bottom of the cell at 3, the pool being maintained by supplying mercury to the pool from line 9 and removing a regulated portion through line H. Fresh chromate solution is fed into the cell through line 12 while a solution which has been subject to electrolysis and which can be designated as used solution is removed through line M.

The mercury removed fromthe cathode through line I I is passed into a vessel 41 suitably supported for vibration by lever 42 supported on bearing 43 and connected to crank 44. In this connection, I have found that when cathode mercury containing chromium amalgam is shaken, almost a complete separation takes place. Chromium amalgam with some excess of mercury comes to the surface as a matted, nonfluid mass. This product has a strong resemblance to flotation froth, and can be recovered by skimming. Excess mercury collects at the bottom of the funnel and can be drawn off, being as fluid as the virgin mercury added to the cell. I have reused mercury separated by this method by placing it back in the cell and found it just as satisfactory as virgin mercury. The efficiency of electrolysis is not at all impaired by the use of mercury recovered in this manner. In the process flow sheet of Figure 1, this is utilized by introducing a shaker between the cell and the mercury filter and combining recovered excess mercury with mercury that has passed through the filter.

The amalgam-mercury mixture from the cell flows into launder 46 while the mercury is taken off at the bottom of th vessel 4| into line H. The amalgam passes to filter 16 to separate the mercury, any excess mercury being taken off through line H. The filter cake is then fed by suitable means indicated by line [8 into a retort [9 wherein a reducin (or non-oxidizing) atmosphere is maintained and the mercury is vaporized and removed through line 2| to a mercury condenser 22, the chromium metal being subsequently removed from the retort. The recovered mercury can be returned through line 23 to line 9 for re-use, if desired. The entire mercuryamalgam stream from the cell can be passed directly to filter 16 or the filter can be by-passed, if vessel 4| is employed.

Used solution from the cell contains small amounts of chromic hydroxide which is filtered through a filter 24 and is recovered as a byproduct. Clear filtrate is returned through line 25 to the plant for further processing.

Example 1 In one operation conducted in accordance with this invention, an electrolyte consisting of an aqueous solution of sodium chromate and sodium carbonate, such as may result from the leaching and subsequent purification of an acidic chrome oxide ore by roasting with soda ash, was placed in the cell 6. The solution in the cell contained 170 grams per liter of sodium chromate and 30 grams per liter of sodium carbonate. The electrolysis was continued for 3%; hours, chromic acid being added to maintain the electrolyte at a pH between 4 and 7. The current density of the cathode was 45 amperes per square foot of cathode surface while the operation was so conducted as to maintain a constant "cell amperage, an average of 2.14 amperes being employed at an average of 4.19 volts. Upon removing the amalgam from the cell, it was found to consist of approximately mercury and 40% chromium metal which was recovered by retorting the amount in a reducing atmosphere to vaporize the mercury. Of the chromium precipitated in the cell, 3.9% consisted of chromium hydroxide and 96.1% consisted of chromium metal which was recovered by the aforementioned final retorting of the amalgam.

In the flow sheet of Figure 2, the alkaline chromate is first converted into chromic acid (as at 3|) by any of the accepted methods, such as precipitation with strong sulfuric acid or electrolysis in a diaphragm cell. Chromic acid is then fed through line 32 to an electrolytic cell provided with a mercury cathode at a rate substantially equal to the rate of electro-deposition of chromium. The starting solution in the cell can be sodium chromate adjusted to an optimum pH for electrolysis between 4 and 7. This pH is maintained throughout the operation by the proper feed rate of chromic acid. Eventually the cell solution may become fouled by impurities, and would then be either discarded or returned to some suitable point in the process. In accordance with this invention another operation was conducted in the following manner:

Example 2 A starting electrolytic consisting of a Water solution of'sodium chromate was placed in the electrolytic cell. Th concentration of sodium chromate was 1'70 grams per liter. A water solution of chromic acid was then added until the pH of solution in the cell became 7.0, following which the current was turned on. The current density of the cathode was 45 amperes per square foot. Cell voltage was maintained at about 4.2 volts, and current was flowing at an average rate of 2 amperes. During electrolysis, th solution of chromic acid was continuously fed to the cell, so that pH of the cell remained between 4 and 7.

Upon completion of the operation, the amalgam was separated from mercury and chromium metal was recovered from the amalgam by distillation. The chromium hydroxide formed was filtered and ignited to chromic oxide. It was found that of the chromium precipitated in the cell, was deposited as metal, and 20% precipitated as hydroxide. Current efliciency calculated on basis of metal formed (neglecting hyis taken off from the bottom of the tank through line IT.

This is an important improvement for instead of passing the entire cathode mercury through the filter one can separate amalgam from mercury prior to filtration, so that only a fraction of cathode mercury and most of the amalgam go to th filter and the excess mercury, freed from most of the amalgam, is delivered back to the cell.

It is desirable to maintain a substantially constant flow of mercury into the cell and a flow of mercury amalgam from the cell for, when the amalgam content of the mercury became too great, the mercury-amalgam mixture ceases to flow.

It is not absolutely essential that the pH of 4 to '7 be observed. However, as the electrolyte becomes increasingly alkaline, the formation and deposit of chrome gradually decreases until they cease. This is shown by the following example.

Example 3 An electrolyte consisting of an aqueous solution of sodium chromate and sodium carbonate, such as may result from the leaching and subsequent purification of an acidic chrome oxide ore by roasting with soda ash, was placed in the cell 6. The solution in the cell contained 170 grams per liter of sodium chromate and grams per liter of sodium carbonate. The electrolysis was continued for 3%.; hours but no efiort was made to maintain the alkalinity in the range of pH 4 to pH 7 and the electrolyte became quite alkaline. The current density of the cathode was 45 amperes per square foot of cathode surface; the operation was so conducted as to maintain a constant cell amperage, an average of 2.14 amperes being employed at an average of 4.19 volts. The electrolyte finally became so alkaline that the formation of chrome was at an end.

Upon removing the amalgam from the cell, it was found to consist of approximately 60% mercury and chromium metal which was recovered by retorting the amount in a reducing atmosphere to vaporize the mercury. Fifteen per cent (15%) of the chromium present in the cell was precipitated of which 25.9% consisted of chromium hydroxide and 74.1% consisted of chromium metal which was recovered by the aforementioned retorting of the amalgam.

While the above operation is not so efiicient, it can be used to advantage to recover chrome metal and provide an alkaline electrolyte which may be treated further, e. g., to recover chromic hydroxide.

I claim:

1. A process for manufacture of chromium comprising electrolyzing an aqueous solution of a water soluble sodium chromate at a pH between about 4 and about 7 between an anode and a mercury cathode to form a chromium mercury amalgam, adding chromic acid to said cell to replenish the chromium deposited and maintain amalgam to vaporize the mercury and leave the chromium as a recoverable metal residue.

2. A process for manufacture of chromium comprising electrolyzing an aqueous solution of a water soluble sodium chromate at a DH between about 4 and about '7 between an anode and a mercury cathode to form a chromium mercury amalgam, adding chromic acid to said cell to replenish the chromium deposited and maintain the solution in said cell at said pl-l, removing said amalgam from said cell, agitating the amalgam to separate free mercury therefrom, separating the free mercury from the residual amalgam, heating the residual amalgam to vaporize the mercury and leave the chromium as a recoverable metal residue.

3. A process for manufacture of chromium comprising electrolyzing an aqueous solution of a water soluble sodium chromate at a pH between about 4 and about 7 between an anode and a mercury cathode to form a chromium mercury amalgam, adding chromic acid to said cell to maintain the solution in said cell at said pH,

removing said amalgam from said cell, heating the residual amalgam to vaporize the mercury and leave the chromium as a recoverable metal residue.

DMITRI VEDENSKY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Hackh, Chemical Dictionary, page 270 (1944).

Liddell, Handbook of Nonferrous Metallurgy, Recovery of the Meta (1945), page 618.

Transactions of the American Electrochemical Society, vol. '7 (1905), pp. to 142; vol. 37 (1920) pp. 479 to 496.

Roscoe and Schorlemmer, Treatise on Chemistry, vol. II, page 1021 (1913). 

1. A PROCESS FOR MANUFACTURING OF CHROMIUM COMPRISING ELECTROLYZING AN ACUEOUS SOLUTION OF A WATER SOLUBLE SODIUM CHROMATE AT A PH BETWEEN ABOUT 4 AND ABOUT 7 BETWEEN AN ANODE AND A MERCURY CATHODE TO FORM A CHROMIUM MERCURY AMALGAM, ADDING CHROMIC ACID TO SAID CELL TO REPLENISH THE CHROMIUM DEPOSITED AND MAINTAIN THE SOLUTION IN SAID CELL AT SAID PH, REMOVING SAID AMALGAM FROM SAID CELL, HEATING THE RESIDUAL AMALGAM TO VAPORIZE THE MERCURY AND LEAVE THE CHROMIUM AS A RECOVERABLE METAL RESIDUE. 